760 DYNAMICAL GEOLOGY. 



6. Mottled Coloring. — 1. Rocks colored red or brownish-red, with oxyd of iron, be- 

 come mottled, through the deoxydation of the iron, by means of waters containing 

 organic matters: the waters often pass through loose sandy beds without altering them, 

 and then reach a clayey layer where they spread and make the changes, p. 695. 



7. External colors due to vegetation. — Minute black, brownish-black, and greenish- 

 gray lichens give an external coloring to rocks, which is often mistaken for their true 

 colors. Outcrops of Granular limestone, a white rock, are usually quite black, from 

 the species with which they are overgrown. Larger lichens sometimes spread over the 

 surfaces of rocks, and give them a mottled aspect. 



Note. — The above observations on the colors of fragmental rocks apply to the 

 decomposed crusts of crystalline rocks, and to some extent to the crystalline rocks 

 themselves. Red, as a color of rocks, always comes from traces of the red oxyd of 

 iron; green is usually owing to disseminated chlorite, but sometimes to serpentine, 

 pyroxene, or hornblende; and black and greenish-black to iron-bearing varieties of 

 hornblende, pyroxene, or mica. 



Granular limestone or marble has often been mottled and veined through an extensive 

 fracturing, and then a displacement of the pieces, and the subsequent tilling of the 

 intervals between the pieces with a deposit of white or colored carbonate of lime. 

 Another style of mottling or clouding in marble is due to the distribution of impurities, 

 the impurities of the original limestone having received a crystallized condition and 

 agreeable colors (being converted into crystalline minerals), during the metamorphism 

 of the rock. 



9. Consolidation of Fragmental Deposits. 



1. Through siliceous solutions, pp. 693*, 725. 



2. Through calcareous solutions, p. 692. 



3. Through the production of an oxyd or silicate of iron, by one of the methods 

 mentioned under section 7. See also p. 695. 



4. Through infiltration of phosphates into calcareous beds, from overlying guano. 



5. By pressure of superincumbent beds, which alone is ineffectual in the case of 

 sand-beds, but may produce some effect with clayey deposits. 



6. Through metamorphism, p. 724. 



II. Crystalline Texture of Rocks. 



1. Through metamorphism, pp 63, 724. 



2. On cooling, from more or less perfect fusion, p. 63. 



3. On depositions from solution, p. 63. (In the case of the opal depositions from hot 

 springs, p. 719, it is questioned whether there is a crystalline texture.) 



4. On passing to the solid state, at the time when made by chemical means, as in the 

 case of beds of gypsum, made from action of sulphuric acid on limestone, p. 234. 



III. Fractures. 



1. By lateral pressure. — 1. The lateral pressure resulting from the contrac- 

 tion of the crust on cooling, pp. 735, 739. 



2. The lateral pressure produced by change of temperature in rocks, p. 700. 



2. By contraction. — 1. Through cooling, producing sometimes a columnar 



structure, pp. 112, 701. 



2. Bv drying, producing sometimes columnar fractures, pp. 84, 701. 



3. By means of foreign substances in crevices or openings. — 



1. The growth of vegetation, p. 607. 



2. Water freezing, p. 674. 



3. Chemical change in the crevice, developing an oxyd of iron or some other mineral, 

 and so prying open and deepening it. 



4. The ice of the bottom of a moving glacier, p. 538. 



